It is known to distribute material, such as a product whose consistency is substantially pulverulent and at most particulate of very small mesh size (less than about 25 microns), on a substrate in a regular, continuous or batch process. For example, in certain techniques, distribution of product on a substrate has been carried out manually and there are known spray devices for similarly distributing a layer of product. The ultimate fabricated item oftentimes requires that the distribution of product be carried out with precision, such that the deposited product, applied by projection or otherwise, have perfect regularity. For example, in the fabrication of semi-reflective glass through an overall manufacturing process including the pyrolysis of a metal salt on the surface of the glass, the metal salt providing the reflective capability need be distributed not only in a thin, but also in a regular layer on that surface. Otherwise, the glass which is intended to reflect at least a portion of the visible or infrared spectrum may fail in its capability of transmission or reflection, or it may exhibit striations or some other irregular feature on or across the surface of the glass.
The aforementioned spray devices, such as a device for spraying a layer of powdered paint, while capable of that stated application, for reason that variations in thickness of the material normally do not detract from the aesthetic appearance finally achieved, could not be used in the present fabrication procedure. Thus, with conventional spray devices employing a circular dispensing orifice or nozzle to aspirate the product by pneumatic evacuation means it is difficult, if not impossible, to deposit a layer of product having constant thickness throughout either the substrate or the area covered by the nozzle. This is because all portions of product projected through the nozzle do not take a single path, but rather a multitude of paths at various angles of inclination relative to the axis of the nozzle. Also, portions of the product move at different speeds in passing to the substrate. Thus, the thickness of the layer varies as the distance from the axis of the nozzle varies.
If a substrate having a large surface area required coating with a layer of product having a constant thickness, quite commonly several spray devices would be resorted to. However, a further problem or difficulty results in the use of a plurality of spray devices, namely that of the overlapping of layers of product from different nozzles. This problem or difficulty has also been found to result from the use of a single spray device, reciprocated back and forth over the surface area of the substrate. Further, the apparatus to provide movement of the spray device may suffer from mechanical problems of wear, and so forth, resulting in downtime in the fabrication process.
It has also been proposed to utilize an apparatus having an exit slit through which product is dispensed. In this apparatus, product in suspension in a gas is communicated to the exit slit by a conduit located generally centrally of the extremes of the exit slit. It has been found difficult, however, to obtain an even, uniform delivery of product over the length of the exit slit and it has been found that the delivery of product in greatest amount has occurred most often within the median portion of the exit slit. Not only does the product move from the exit slit to provide an unequal distribution on the substrate throughout the area of the exit slit, the apparatus is subject, also, to considerable load loss, so that the flow of product is not uniform along the exit slit.